2.1: Lines
- Page ID
- 232
1. Lines (definitions)
Everyone knows what a line is, but providing a rigorous definition proves to be a challenge.
Definition: Line
A line with slope \(m\) through a point \(P = (a,b)\) is the set of all points \((x,y)\) such that
\[\dfrac{y-b}{x-a}= m.\]
2. The Slope Intercept Form of the equation of a Line
Given a point \((x_1,y_1)\) and a slope \(m\), the equation of the line is
Definition: Slope Intercept Equation of a Line
\[y-y_1=m(x-x_1)\]
3. Piecewise Linear Functions
A function is piecewise linear if it is made up of parts of lines
Example 1
\[f(x)=\begin{cases} x+4 & \text{if }x\leq-2 \\ 2x-1 & \text{if } -2<x<1 \\ -2x & \text{if } x\geq1\end{cases}\]
We graph this line by sketching the appropriate parts of each line on the same graph.
4. Applications
Example 2
Suppose you own a hotel that has 150 rooms. At $80 per room, you have 140 rooms occupied and for every $5 increase in price you expect to have two additional vacancies. Come up with an equation that gives rooms occupied as a function of price.
Solution
Let \(x\) be the price of a room and \(y\) be the number of rooms occupied. Then we have an equation of a line that passes through the point \((80,140)\) and has slope \(-\frac{1}{5}\). Hence the equation is:
\[y - 140 = -\dfrac{1}{5}(x - 80)\]
or
\[y = -\dfrac{1}{5} x + 16 + 140\]
or
\[y = -\dfrac{1}{5} x + 156.\]
Exercise 1
What should you do if your two year old daughter has a 40 degree C temperature?
Hint: We have the two points: \((0,32)\) and \((100,212)\).
Exercise 2
Suppose that your company earned $30,000 five years ago and $35,000 three years ago. Assuming a linear growth model, how much will it earn this year?
Exercise 3
My rental was bought for $204,000 three years ago. Depreciation is set so that the house depreciates linearly to zero in twenty years from the purchase of the house. If I plan to sell the house in twelve years for $250,000 and capital gains taxes are 28% of the difference between the purchase price and the depreciated value, what will my taxes be?
Exercise 4
Wasabi restaurant must pay either a flat rate of $400 for rent or 5% of the revenue, whichever is larger. Come up with the equation of the function that relates rent as a function of revenue
Larry Green (Lake Tahoe Community College)